Aromatic amines are important intermediates which need to be produced inexpensively and in large volumes. Production plants for aromatic amines are therefore generally built for very large capacities. The high productivity of these plants is ensured by very long reaction cycles and uninterrupted running between the start-up and shut-down procedures of the hydrogenation for regenerating the hydrogenation catalysts employed.
The main field of application of 2,4-diaminotoluene is the production of toluene diisocyanate (TDI). It is produced industrially by the hydrogenation of 2,4-dinitrotoluene.
The main field of application of aniline is the production of methylenediphenyldiamine (MDA) which is used to produce methylenediphenyldiisocyanate (MDI). Aniline is generally produced on an industrial scale by catalytic hydrogenation of nitrobenzene with hydrogen. It is particularly preferable to conduct the reaction as described in EP 0 944 578 A2 (isothermal mode of operation) and in EP 0 696 574 B1, EP 0 696 573 B1 and EP 1 882 681 A1 (adiabatic mode of operation). The production of MDA is described in numerous patents and publications (see, for example, H. J. Twitchett, Chem. Soc. Rev. 3(2), 209 (1974), M. V. Moore in: Kirk-Othmer Encycl. Chem. Technol., 3rd. Ed., New York, 2, 338-348 (1978)).
What is common to the described isothermal processes for producing aniline is that the starting material nitrobenzene is vaporized at elevated temperature in the hydrogen stream.
The reaction is generally conducted such that the gaseous nitrobenzene/hydrogen mixture is passed into the hydrogenation reactor and reacted here over the fixed-bed catalyst, optionally with a downstream post-reactor, at elevated temperature and atmospheric pressure.
The heat of reaction liberated is removed from the reactor via a heat exchanger and generally used for steam generation.
The reaction products aniline and water exit the reactor in gaseous form and are condensed out of the hydrogen stream via multistage condensation. The excess hydrogen is recirculated, supplemented with fresh hydrogen and again vaporized together with nitrobenzene and passed into the hydrogenation reactor as a mixture.
The hydrogen becomes loaded with gaseous impurities on account of the recirculating mode of operation. To remove these impurities, a substream is withdrawn from the hydrogen circuit and incinerated in the thermal exhaust air purification step.
The condensed-out reaction products separate into an organic phase (crude aniline) and an aqueous phase (aniline water) and these are subjected to further processing separately. The crude aniline also comprises water and organic by-products in dissolved form which are separated off by distillation.
Initially, a column is used to distil off the low-boiling secondary components (for example cyclohexylamine, cyclohexanone, benzene) overhead and the water in the sidestream as an aniline-water azeotrope. The sidestream is biphasic and is recycled into the abovementioned phase separation.
The low-boiling secondary components which also comprise aniline are drawn off at the top of the column and can either be disposed of directly in an incineration plant or initially condensed and later incinerated together with other residues.
The bottom product (aniline+high-boilers) is freed of the high-boiling by-products (for example N-cyclohexylaniline, N,N-diphenylamine, phenol) in a second distillation column. The pure aniline is distilled off overhead. The high-boilers accumulate in the bottom and are further concentrated in a third distillation column (residue column).
Aniline is recovered at the top of the residue column and fed back into the second column together with the bottom product of the first column. The bottoms from the residue column are transferred into a residue container. Here, a residual aniline content serves as solvent to maintain the pumpability of the residue. In addition, the residue is stored at elevated temperature to avoid precipitates or excessively high viscosities. The high-boilers together with the diluent residual aniline are supplied from the residue container to an incineration step.
The water separated in the phase separation is freed of the dissolved aniline by distillation and fed as wastewater into the biological wastewater treatment plant of the site. The aniline is distilled off as an azeotrope with water and is recycled into the abovementioned phase separation.
All stages of the process for producing aniline are carried out in a continuous operating mode.
The performance of a process for hydrogenating aromatic nitro compounds is defined by the quality of the product. The performance of a hydrogenation process is also defined by the ability to operate the entire process continuously without significant production outages. The shutting down of the hydrogenation, the regeneration of the hydrogenation catalyst and the starting up again of the hydrogenation process are generally referred to as a smooth sequence of events in the hydrogenation cycle.
Since all stages of the process for producing aniline are carried out in a continuous operating mode during the hydrogenation cycle, it is also necessary to operate the work-up of crude aniline without interruption. Lastly, it is also important to achieve a high yield of the desired product which means avoiding by-products in the reaction and minimizing production losses in the plant. Such losses arise in the distillation of the product for example, since concentrating the high-boiling secondary components in the bottom is possible only to a certain extent without the residue solidifying or parts thereof precipitating and forming undesired precipitates. A significant proportion of product is therefore always incinerated with the secondary components.
EP 0 794 170 A1 discloses a process for separating off high-boilers in the production of diaminotoluene. The crude dewatered diaminotoluene is passed into a distillation unit comprising a packed column having a continuous evaporator for the column bottom. In one embodiment, (FIG. 1) the packed column has a falling-film evaporator arranged downstream of it into which the bottom discharge of the packed column is passed. In another embodiment (FIG. 2), a falling-film evaporator is arranged upstream of the packed column. Here, the packed column is fed from the vapors of the falling-film evaporator. In both cases, the falling-film evaporator is intended to ensure that very little of the product of value (meta-diaminotoluene) is lost together with the high-boiling residues. In both cases, the bottom product of the falling-film evaporator (consisting essentially of high-boiling residues) is mixed with a condensed ortho-diaminotoluene stream withdrawn in the upper part of the packed column.
EP 0 696 574 B1 discloses in examples 9 and 10 a process in which the product of nitrobenzene hydrogenation including the water of reaction is distilled in a distillation column and the bottom product of the distillation column is diluted with the aniline-rich phase obtained after phase separation of the condensate of the top product of the same distillation column.
EP 1 005 888 B1 describes a rinsing apparatus for removing residues from the bottoms outlet of an evaporation apparatus and the use of said rinsing apparatus for the distillative work-up of salt-containing solutions. It is a disadvantage that the used rinsing agent again entails a certain cost and inconvenience to the extent that it needs to undergo costly incineration. Moreover, such a rinsing apparatus is unsuitable when high temperatures prevail at the bottom of the column and therefore high-boiling solvents would need to be used or the bottom would need to be cooled in order to carry out the rinsing operation without vaporization of the rinsing agent. In the process described, preference is given to using water, which, however, is only of limited suitability as a washing agent for organic residues. In addition, it is not always possible in practice to avoid washing agent entering the column and impairing the quality of the top product and/or the further work-up of the top product.